Testing and protection of electrical distribution systems



Jan. 18, 1949. w R. SCHEIRMANN 2,459,136

NOW BY CHANGE OF NAME R. SHERMAN TESTING AND PROTECTION OF ELECTRICALDISTRIBUTION SYSTEMS Filed July 19, 1943 s Sheets-Sheet 1 FIE) ll j Jan.18, 1949. R, SCHEIRMANN 2,459,186-

NOW BY CHANGE OF NAME R. SHERMAN TESTING AND PROTECTION OF ELECTRICALDISTRIBUTION SYSTEMS Filed July 19, 1943 3 Sheets-Sheet 2 INVENTOR.

Jan. 18, 1949. 5 NN 2,459,186

NOW BY CHANGE OF NAMER. SHERMAN TESTING AND PROTECTION OF ELECTRICALDISTRIBUTION SYSTEIS Filed July 19, 1943 3 Sheets-Sheet 3 Patented Jan.18,1949

TESTING AND PROTECTION OF ELECTRICAL DISTRIBUTION SYSTEMS RaphaelScheirmann, Warren, Ohio, now by change of name Ralph ShermanApplication July 19, 1943, Serial No. 495,311

Claims. 1

This invention relates to means for permanently or intermittentlysupervising, testing, indicating and controlling the condition ofcontact between electrically conductive parts which are required tomaintain intimate contact between them, and this without interruption ofthe operation of the devices or installations which these parts belongto.

This invention is applicable to contacts between contacting parts of anykind provided they are capableof conducting electrical energy, but

. it is particularly valuable in application to systems traversed by aworking current i. e. 9. current doing useful mechanical, electrical orchemical work and liable to be disturbed or injured by fundamentallyimperfect or suddenly or gradually deteriorating contact between twocontacting current-carrying parts.

It is one of the objects of this invention to provide means whereby theoperator of an electrical installation is warned of any imperfection ordeterioration of contact in the circuit as soon as it arises, thusenabling him to remedy the defeet before it has resulted in injury tothe inwhich they are operating. The oxygen or the moisture in the airwill cause the formation of an oxid layer on. contact surface. Dust, oilor grease may form insulating coatings thereon.

Oxidation may also occur through the action of heat. Loosening ofmechanical connections may occur through many causes. In all such casesthere results a rise of resistance to the passage of current.

According to this invention the variations in this intermediateresistance are rendered visible or audible in indicating or alarmdevices, or are utilized to activate switches, permanently connected tothe contacting parts to be protected or supervised, while these partsare traversed by the working current. In any case the attendants oroperators are thus warned and induced to remedy the defect or, ifnecessary, to temporarily throttle or cut out the current.

All variations of the intermediate resistance 'between contactingsurfaces result in corresponding variations of the voltage drop at thesesurfaces. This is particularly true in the case of high currentinstallations such as are present in electric melting furnaces operatingwith currents up to and beyond 50,000 amp. Obviously in such cases anyunnoticed rise of the intermediate resistance in a contact may causevery considerable injury to the parts and losses of energy and/or metal.Frequently the electrodes are overheated to the extent of becomingincandescent, arcs may form between the electrodes and their holders,the bus bars may be heated to melting temperature etc. In most cases thedefect that is at the root of all this, will not or cannot be detectedin time and the installation may be out of working for many hours oreven days.

Hitherto no means have been available for preventing with certainty andautomatically the damage arising from such causes.

Numerous devices are on the market for measuring the specific resistanceof solid conductors, but these devices are not suitable for the purposehere in view, i. e. the detection of imperfect contact betweencontacting parts traversed by a working current. For, in contrast to theresistance in solid conductors, the intermediate resistance betweencontacting parts varies owing to circumstances such as set out above andalso to the intensity of current. If this resistance is ascertained ormeasured under current conditions considerably below those of the normalworking current, the results may be deceptive. Therefore in thesemeasuring devices, all of which operate with a separate current,frequently as high a current intensity is used as possible, but sincethey must be portable, the intensity is limited for technical reasons toa few hundred amperes. Obviously now it is impossible, with so low acurrent, to correctly measure contact resistances in apparatus, forinstance electric melting furnaces, traversed by many thousand amperes.But quite apart from this technical limitation, these devices are onlyused to measure resistances in a de-energized equipment. They do notindicate all conditions arising during actual operation, when vibrationsor temperature influences or some other circumstances may altogetherchange the resistance at the contacts, sometimes followed byarcformatlon and eventual melting down of contacting parts.

In contrast to these existing methods of resistance measurement, thepresent invention enables me to test the conditions of contact in theequipment while it is operating and traversed by the working current, Ican provide for a continuous testing and measuring and an automaticsignalling of contact conditions and variations, and I may also providemeans for automatically throttling or cutting out the current. In everycase I utilize the circumstance that any deterioration of contact isaccompanied by a corre-,

sponding change of voltage drop at the contact.

In cases where the amperage is subject to considerable variation, Iprefer to use a measuring instrument indicating the quotient of theprevailing system current and voltage, for instance a cross-coilinstrument, which is known to indicate directly the resistance, forindicating the disturbance.

While as a rule the working current will be used to actuate theindicating instruments, alarms or switches, I may also use a separatecurrent for this purpose without in any way impairing the result, whichwill be the same as that obtained with the aid of the working current,since I employ the separate current while the working current fiows.

For instance, if a separate direct current is used in an alternatingcurrent system to indicate the condition of contacts in accordance withmy invention, the direct current voltage drop arising from theintermediate resistance exactly corresponds to the actual intermediateresistance formed at this moment by the entire working current. Thus theseparate direct current used for testing and indicating these variationsfurnishes the same picture of the conditions of operations, as would beobtained with the aid of the full working current of thousands ofamperes. This method may be particularly advantageous whenever it isdesired to eliminate the induction effect exerted on the test(measuring) results. In all cases the operator, without being requiredto take action for this purpose, is continuously furnished fullinformation regarding any deterioration of the contacts during operationand is thus enabled to prevent in time injury to, or destruction of, theequipment.

I will now proceed to describe by way of example some characteristicapplications of my invention in order to thereby show that it will beuseful in widely different fields of utilization of electrical energy.The examples hereinafter described with reference to the drawings shouldnot in any way be considered as limiting the scope of my claims to theindividual means or combinations of means nor to the particular uses andapplications shown and described. For there exists hardly any form ofelectrical installation in which my invention could not be applied withadvantage. Certain features of my invention disclosed herein but notclaimed are claimed in my copending application Serial No. 55,605, filedOctober 20, 1948.

Throughout the drawings affixed to specification and forming partthereof, similar numerals are intended to designate similar parts. Inthe drawings Fig. 1 is a diagram of an alternating current systemcomprising an electrical melting furnace with an electrode mounted in aholder, in combination with means according to this invention forcontinuously supervising, indicating and controlling the variations ofcontact between these parts during operation, making use of the work ingcurrent for effecting this indication and control.

Fig.2 is a similar view of a direct current system in combination withmeans for supervising contact conditions in the bus bars by means of theworking current.

Fig. 3 is a diagram of an alternating current system with a bus barcontact controlled by means of a separate direct current.

Fig. 4 is a similar view of a direct current system in combination withmeans for supervising a bus bar contact by means of a separatealternating current.

Referring to the drawings and first to Fig. 1 this is a diagram ofconnections of a circuit feeding alternating current of high intensitythrough the lead I to the electrode holder 2 of the electrode 3 of anelectric melting furnace and through it to the body 5 of molten metal.The condition of contact between the electrode and the holder issupervised and controlled continuously, while the furnace is inoperation, or while the supply circuit is connected to a useful load, byproviding a measuring instrument which may be permanently connected inthe circuit and at any time indicates to the operator Whether thecontact is good or requires improving. To this end two wires 8 and 9,connected to the holder and the electrode at 5 and l, respectively, leadto the primary winding ii of the voltage transformer id. The potentialof the secondary transformer. winding i2 is transmitted by wires 13 andI l! to the bridge rectifier 22, while on the other side a wire it leadsto the contact piece I47 of a small controller whose normal position ismarked by the letter A, where the current from contact piece It! passesthrough the contact segment l5, connecting wire it, segment i'i, contactpiece I53 and wire 2| to the other side of the rectifier 22 and from oneside of the rectifier through wires 23, I43, 25 to the voltage coil 25of the measuring instrument (cross-coil ohmmeter) 26, from the otherside through Wire I63, contact 21 and wire 28 to the series resistance29 allowing adjustment to different measuring ranges, and from thisresistance through the brush I44 and wire 3| to the second end of thevoltage coil 25 of the measuring instrument.

The electrode 3 is supplied with current through wire I54, the primarywinding 33 of the current transformer 32, whose secondary winding 34 isconnected on one side to the rectifier 42 by way of wire 35, amperemeter33 and wire 4!, while the other winding is connected to the rectifier bythe wire 43. From this rectifier the wire 41 leads to one end of theadjustable shunt 48, while a Wire 49 connects the rectifier to the brush50 of this shunt, whose ends are connected on one side through wiresI55, to one end of the current coil 53 of the cross-coil instrument, onthe other side through wires 5|, 52 to the other end of this coil.

Since the cross-coil ohmmeter directly indicates the ratio of potentialand current, the resistance can be read directly on its scales.Adjustable series resistance 29 and adjustable shunt 48 are provided forthe purpose of adapting the instrument to various conditions indifferent installations.

Since this ohmmeter merely indicates the ratio of potential and current,the amperage fed to the furnace is immaterial to the result. As long asthe intermediate resistance between the furnace electrode and the holderremains constant, its pointer I5I will remain stationary. Whenever thecontact should deteriorate by a loosening of the electrode in the holderor from other causes the pointer will move at once, and if thedeterioration of contact reaches a certain limit, the pointer willestablish contact between the terminals 54 and 59 of a signallingcircuit which comprises the current source 58. one terminal of which isdirectly connected through wire 51 to the relay 55, while the otherterminal is connected to the relay through wire 58, closed contacts 59,54 and wire I55. Relay 55 will now close the contact 54 and therebyexcite, with the aid of current supplied by the current source 55, theoptical oracoustical signalling device 55. Thus the operator, unless hehad ascertained the deterioration of contact by observing the positionof the pointer of the ohmmeter, is warned at once of the dangerthreatening the furnace and its contents. If he should not heed thesignal or if he should be absent, the signalling instrument will, bymeans of a time-relay, resistance and circuit breaker (not shown)connected to it by the wires 51, 58 first throttle the current supplyand thereafter cut it out altogether, An excess voltage which may ariseoccasionally, is provided for by a switching relay I 38 connected inparallel to the voltage coil 25 of the cross-coil instrument by a wireI3I leading to the regulating brush I44, the other wire I32 beingconnected between wires I43 and 23. Any excess voltage in the potentialcoil of the instrument will cause the relay I38 to open the contact 21.Further means serving as a protection against excess voltage will bedescribed farther below.

In the case of extremely low voltage it is advisable to ascertain fromtime to time whether the contacts in the testing circuit are perfect. Tothis end a rotatable controller is provided with a separate alternatingcurrent source feeding a low tension transformer 18 with a resist ance13 regulating the current output of the secondary winding 12 which isconnected by the wire 14, amperemeter' 15 and wire 15 to the contactpiece I55, while on the other side the wires 18 and 19 lead to thecontact piece I55. Between the wires 14 and 18 a voltmeter 11 isconnected for measuring the voltage output.

When the controller is turned into the first testing position B, currentwill flow from one terminal of the low tension transformer 18 throughcontact piece- I55, segment 82, wire 83, segment 84, contact piece I48,wire I49 and wire I3 to one end of the winding I2 of the voltagetransformer I8. The other end of winding I2 is connected to the sourceof current by means of the contact piece I56, segment 81, wire 85, se

ment 85, contact piece I41 and wire I4. Hereby this winding I2 isexcited and a voltage is induced in the other winding II, which, if allthe contacts are in good order, is short circuited through wire 9,contact 1 of the electrode 3, wire 8 and the contact 5 of the electrodeholder 2. The deflection of the ointer of amperemeter 15 at apredetermined deflection of the voltmeter 11 then allows to readilyascertain whether the contacts 1 and 5 at the electrode and its holder vare in order. If they should be oxidized, a mateis exactly the same asin Fig. l.

' (check up device in position B").

'rially lower current will pass through. In every installation theempirically determined values are ascertained by tests. If thedeflection of the amperemeter is found to be too low, the voltage at theregulating resistance 13 is increased until the instruments show thatgood contact is reestablished.

By setting the small controller to the position C, the other'side of thecircuit of the voltage coil is then tested. To this end one pole of thevoltage transformer is connected with one side of the rectifier 22 byway of contact piece I55, segment 88, wire 89, segment 98, contact pieceI58 and wire 2I. The other pole is connected with the other side of therectifier 22 by way of contact piece I55, segment 93, wire 92, segment9I, contact piece I48, wire I49 and wire I4I.

By applying a predetermined voltage and varying it any deflection of thepointer I5I of the cross-coil instrument 25 will be noticed, while thecurrent cell 53 is fed by the working current through the voltagetransformer 32.

The check-up device hereabove described can be used also with advantagefor testing any other part of the apparatus.

While Figure 1 illustrates an electrode and electrode holder of afurnace, I wish it to be understood that this is merely one example outof the great number of points of contact in an alternating currentinstallation which can be controlled and protected in the manner heredescribed.

Figure 2 illustrates the application of the principle underlying thisinvention to a direct current system, the testing device being againtraversed by the working current. Here the contact between two bus bars28I, 282 shall be tested.

The greater part of the equipment including.

the wiring from the cross-coil instrument and its voltage coil up to thecontact 21 and wire I43 Contact 21 is connected by way of wires 285 and285 to the contact 283 on bus bar MI. The other side of the voltage coilis connected to the other bus bar 282 through wires I43 and 2I8, contactpiece 2I3 (the controller being in the normal operating position A),segment 2I2, wire 2| I, segment 2I8, contact piece 289, wire 281 andcontact 284.

This inner connection of the current coil 53 up to the adjusting shunt48 is also the same as in Fig. 1. 7 One end of the shunt 48 is connectedthrough the wires 233, 232 to one end of the shunt 23I which is insertedin the lead 298 to bus bar 28L The brush 58 is connected through wire238, contact piece 225, segment 224 (the controller beingin the positionA), wire 223, segment 222, contact piece 22I, wire 228, amperemeter 291and wire 2I9 to the second terminal of the shunt 23 I.

This system functions exactly like the one illustrated in Fig. 1. Herealso any excessive rise of the intermediate resistance will cause thepointer of the cross-coil instrument to be deflected so far as to closethe contact 54 and 59, whereby the signal device 55 is actuated. Theprotection against excess voltage is the same as in Fig. 1 and here alsotwo positions of the check- 'up device are shown, in which the contactsare tested. Y

This example comprises a separate source of direct current 234 with avoltage divider 235, One pole is connected to wires 242, 243, contactpiece 245, segment 252, wire 253, "segment 254, contact piece 289 andwire 281 to the contact 284 on bus bar 202. The other pole from theseparate direct; current source (battery) is connected through contactpiece 24!, segment 241, wire 248, segment 249, contact piece 250 andwires 25!, 205 to the contact 203 of bus bar Here, as in the systemshown in Fig. 1, the deflections of amperemeter 238 and the voltmeter29! of the separate source of current will show whether there exists anintimate contact in the tested system. Here also, if necessary, thevoltage will be increased until the contacts are reestablished.

In the position C the connection leading to the shunt 23! is tested.Since this is done in exactly the same manner as explained withreference to position B, there is no need for going into the details ofthis connection.

Obviously here also the check-up device can be used for testing anyother contacts in the sys-,

tem

Fig. 3 illustrates an alternating current system comprising bus bars,the contacts between which are placed under supervision of devicessupplied with separate direct current.

This direct current circuit comprises a lowtension source of directcurrent for instance a battery 303. feeding direct current through theregulating resistance 304, wire 305, amperemeter 306 and wire 30'! tothe bus bar 30!. From the other pole of the battery current flowsthrough wire 30B, shunt 309, wire 3i0, regulating inductive resistance(choking coil) 3!! and Wire 1H2 to the other bus bar 302.

From shunt 309 testing wires 3!'!, 3l8 lead to the adjustable shunt 48and its brush 50. The shunt is further connected, in exactly the samemanner as in the examples described above, to the current coil 53 of thecross-coil instrument 26. The voltage coil of this instrument isconnected, exactly as in the previous examples, up to the contact 2'!and wire I43. This wire is connected through wire 3H5 to bus bar 302,while contact 2'! is connected through the regulating choking coil(inductive resistance) 3!4 and wire 3 l 3 to the other bus bar Theregulating resistance 304 regulates the direct current used for testing.Its amperage is indicated by the amperemeter 305. Shunt 309 feedscurrent to the current coil 53 of the crosscoil instrument.

The drop of potential, caused by this direct current, at the point ofcontact of the two bus bars is fed through the wires 3!5, 3!B to thevoltage coil 25 of the cross-coil instrument.

The entire system operates exactly as described before with reference toFigs. 1 and 2.

Obviously the working current (alternating current) passing through thetwo bus bars 'at the contact point will suiler a drop of potential also.In order to prevent this drop of potential of the alternating currentfrom influencing the testing apparatus, the two choke coils 3H and 3!4mentioned above are provided. They offer a high inductive resistance tothe passage of the alternating current, while allowing the directcurrent to pass through freely, and therefore exact tests can beobtained during operation with the separate direct current withoutpaying regard to the presence of alternating current. The choke coils3!! and 3!4, thus serve as iso-, lating units for in effect isolating orsubstantially separating the measuring circuit from the load circuit,viz., the direct-current circ it from the alternating-current circuit.

The drop of potential of the separate ir current is exactly proportionalto the intermediate ohmic resistance ofiered to the entire alterhatingworking current. Even in those cases where the direct current intensityonly amounts to a small percentage of the alternating working currentintensity, a correct picture of the intermediate resistance is obtained.

This mode of testing this resistance offers particular advantages in allcases where the effects of an inductive resistance at the contact pointto be controlled shall be eliminated. When using alternating current forthe tests, this inductive resistance might make the drop of potentialappear much higher than would correspond to the purely ohmic resistance.The use of separate direct current eliminates this phenomenon and alsooffers the advantage of allowing to ascertain the presence of fairlygood contact even before operations are started, while any variation ofthe intermediate resistance during operation is indicated also.

The separate source of direct current, such as the battery, can becharged up from the alternating current system with the aid of acorresponding low-tension transformer and a rectifier.

In Fig. 4 a direct current system is illustrated in which the testing isdone at a pair of bus bars during operation with .the aid of separatealternating current. An alternating current source supplies currentthrough a low-tension transformer 404, whose secondary winding 406 isconnected at one end to the regulating resistance 401, wire 408,amperemeter 409, and through wire 4!!) to the bus bar 40!. From theother end of the secondary transformer winding 406 current flows throughwire 4!! primary winding 33 of the current transformer 32 and wire M2 tothe second bus bar 402.

The secondary winding of the current transformer 32 is connected to thecurrent coil of the cross-coil instrument through the rectifier in thesame manner as has been explained with reference to Fig. 1, as caneasily be ascertained from Fig. l, where the same parts are marked withthe reference numerals already used in V Fig. 1.

The drop of potential at the point of contact caused by the alternatingcurrent is passed on to the primary winding of the voltage transformerl0 through wires M3, 4 I 4. The secondary winding !2 carries thisvoltage through the rectifier 22 to the voltage coil of the cross-coilinstrument in exactly the same manner as in Fig. 1.

For the sake of simplicity the check-up device has been omitted in Figs.3 and 4.

The amperemeter 409 indicates the amperage used in this testing.

The drop of potential of the working direct current at the bus barcontact occurs parallel to that of the alternating current, but thisdrop of potential of the direct current cannot influence the testresults. The transformers I0 and 32 transform and thereby transfer thedrop of potential of the alternating current and the intensity of thiscurrent onto their secondary windings and further to the cross-coilinstrument. In contradistinction thereto the transformers prevent thedirect-current drop of potential in the network from reaching thesecondary windings. Thus here, as in Fi 3, t e tests with separatealternating current are not disturbed by the working current. In thisinstance the transformers I0 and 32 serve as the isolating units forseparating the measuring circuit from the load circuit. If precisepreservation of the instrument transformer ratio, and highly' ac-vcurate indications are desired, suitable blockin devices such ascondensers or rectifiers connected in opposition to direct-current flowmay be interposed in the circuits of the input windings II and 33 of theinstrument transformers, as will be well understood by those skilled inthe art.

This mode of testing the contacts will be particularly advantageous incases where the contacts are liable to frequently deteriorate materiallyor to even open altogether, since instead of the full network voltagefor instance of 110 or 220 volt, only a few volts from the separatesource of current can reach the test instrument and it is easier toprotect the instruments from injury by this low voltage.

To also illustrate the use of separate alternating cm'rent for testingthe contacts in an alternating current system does not appear tobewarranted since the combination of apparatus will be the same as shownin Fig. 4.

For instance in a system of this kind operating with many thousands ofperiods, separate alternating current of 25 or of 60 periods may be usedfor measuring and indicating the variations of intermediate resistance.The inductive resistance of the measuring transformers which aredesigned for a low number of periods, oppose so high an inductiveresistance to the high frequency alternating current that any influencevmethods, the present invention is not primarily,

concerned with the actual magnitude of the ohmic resistance of acontact, the main purpose being to let the operator know when adeterioration has taken place which renders it necessary for him to takepreventive action.

If a protective equipment according to this invention is used inconnection with an alternating current system operating with very highcurrent intensities, for instance in a circuit comprising an electricmelting furnace or a high-duty transformer, it may happen that thetesting and signalling lines must be located in the vicinity of verystrong magnetic fields whereby they will be exposed to a high inductioneffect.

In order to paralyze this effect, the wires will be arranged as closelytogether as possible in order to keep the induction effect as uniform aspossible. It may nevertheless happen that the induction 'will not beexactly the same in two wires leading to a protective device, and insuch a case an additional voltage may be induced in the measuring wires,which may even amount to a multiple of the normally measured voltagedrop. I prefer connecting with each other the ends of each pair ofmeasuring wires and to connect to their opposite ends leading to theswitchboard a sensitive alternating current voltmeter or a moving coilvoltmeter with a rectifier with or without a voltage transformer, orsome other sufficiently sensitive indicating instruments. If in a Inorder to eliminate this induction effect normal test the two wires showa difference of inductive potentials, one of them is reduced orincreased in length until its induction potential is equal to that ofthe other wire and'is thus compensated. The indicating instrument isthen replaced by the regular testing device.

Obviously the cross-coil instruments shown and described throughout thespecification may be replaced by other suitable measuring or indicatinginstruments, for instance a wattmeter or an amperemeter in combinationwith, or without, a series resistance.

The examples of applications of this invention in the protection ofelectrical systems show that this protection can be obtained with theaid of the working current in direct and in alternating current systems.The invention is further applicable to conductors and to entire parts ofa network. The devices to be used for protection may be stationary orportable and of a merely indicating (signalling) or recording type andmay be provided with amplifying devices. The term "electrical system isemployed in the description and claims to designate an electricalinstallation under load or energized. For example, the term refers toelectrical installations where the current from the network, generator,battery, etc., or any .other source of power is taken with the mainpurpose of producing work or converting electrical energy to anotherform of energy, for instance, conversion into mechanical work or heat,or another form of electrical magnetic or electromagnetic energy as inhigh frequency induction heating. The term electrical system as hereinused, excludes purely measuring circuits where the only source ofcurrent'applied is a separate source used with the purpose of makingmeasurements without performing work in some manner as explained above.

01 the equipments to be protected by means of this invention thefollowing are particularly noteworthy: bus bars, connections in motors,generators, transformers, fuse connections, cable connections withinmachines and apparatus, electric melting furnaces etc. and in a generalway power stations, transformer stations, steelworks, chemical works,electric railway systems, electrochemical installations for the recoveryof metals such as copper, nickel, silver etc., but also electricalcooking, heating and illuminating installations. I

While the principal and most important field of application of thisinvention is the protection of electrical systems, i. e. systemstraversed by a working current, it will be apparent that the inventioncan be used also with similar advantage in the protection of devices,apparatus and machinery of non-electrical character in which twoseparate electrically-conductive parts are jointed together and held incontact with each other by mechanical means such as rivets, screw boltsor the like or by soldering, brazing or welding. In most types ofworking machines, prime movers etc. screw bolts jointing two contactingparts may become loose during operation under the influence ofvibrations or the like. In aircraft certain parts such as bracingmembers may be loosened through the impact on the ground when landing.

The safe operation of electrical installations on board of ships, moreespecially warships may depend on the automatic supervision of thecontacts by means of this invention. In all these cases a low tensioncurrent traversing the joint or contact will be aifected by the rise ofthe intermediate resistance resulting from such loosening, and this riseof resistance can be rendered visible or audible during operation withthe aid of my protection devices.

The control efiected according to this invention includes, besides thechanges in the ohmic resistance, also the changes in the inductive(apparent) resistance in alternating current systems.

The term every essential magnitude" is intended to include allresistance changes occurring between the normal state of the contactsand a degree of deterioration which involves danger to the functioningof the system.

I wish it to be understood that I do not desire to be limited to theexact details disclosed in the specification and drawings, for obviousmodifications will occur to persons skilled in the art.

I claim:

1. A protective arrangement for rendering perceptible predetermineddegrees of changes of contact conditions in an electrical system havinga circuit traversed by load current including a pair of conductors incontact, said protective arrangement comprising in combination with suchcontact and connections for connecting said contact to a useful load andfor causing passage of load current through such contact, a deviceresponsive to ratio of voltage to current having a movable element, atranslating device actuated by movement of said movable element, saidratio responsive device including a voltage winding and a currentwinding, connections bridging the voltage winding across said contactandadapted to be connected permanently and connections having a serialrelation to said contact for energizing the current winding inproportion to the current traversing said circuit and adapted to bepermanently connected.

2. A protective arrangement for rendering perceptible predetermineddegrees of changes of contact conditions in an electrical system havinga circuit traversed by load current including a pair of conductors incontact while connected to a useful load, said protective arrangementcomprising in combination with such contacting conductors andconnections for causing passage of load current through such contactbetween the contacting conductors, an instrument responsive tovoltage-current ratio having a movable element, said ratio responsiveinstrument including a voltage winding and a current winding,connections vfrom the voltage winding across said contact, andconnections having a serial relation to said contact for energizing thecurrent winding in proportion to the current traversingsaid circuit.

3. An electrical system comprising in combination a current-carryingcircuit and a protective arrangement therefor, said circuit including apair of conductors in contact, the contact impedance of WhiChlS subjectto variation, and a useful load supplied by said circuit, saidprotective arrangement comprising apparatus for indicating the magnitudeof such contact impedance, said apparatus comprising in combination withsuch contact and connections for causing the passage ofalternating-current through such contact a voltage-current ratioresponsive instrument including a voltage winding and a current winding,connections including a rectifier operatively connecting the voltagewinding across said contact, and connections including a secondrectifier having a serial relation to said. contact for energizing thecurrent winding in proportion to the alternating current traversing saidcontact.

4. In an electrical system including a circuit traversed by a loadcurrent and containing an impedance element, the impedance of which issubject to variation, an apparatus for indicating the magnitude of suchimpedance, said apparatus comprising an electrical instrument responsiveto ratio of voltage to current including a voltage winding and a currentwinding, an auxiliary source of current having connections to saidimpedance for causing measuring current-to traverse said impedance,connections from the voltage winding across said impedance element, andconnections having a serial relation to said impedance element forenergizing the current winding in proportion to the current traversingsaid impedance element, each of said instrument connections includingisolating units for substantially eliminating load current from thewindings of said ratio instrument.

5. In an electrical system including a directcurrent circuit traversedby a load current and including a resistance the magnitude of which issubject to variation, apparatus responsive to the magnitude of suchresistance, said apparatus comprising a ratio responsive instrumentincluding a voltage winding and a current winding, a local source ofalternating current, connections for causing said alternating current totraverse said resistance, connections from the voltage winding acrosssaid resistance, and connections having a serial relation to saidresistance for energizing the current Winding in proportion to thecurrent traversing said resistance, said instrument connections eachincluding electrical isolating units for substantially eliminatingdirect load current from said windings.

6. In an electrical system including a directcurrent circuit traversedby a load current and including a resistance the magnitude of which issubject to variation, apparatus responsive to the magnitude of suchresistance, said apparatus comprising a ratio responsive instrumentincluding a voltage winding and a current Winding, a local source ofalternating current, connections for causing said alternating current totraverse said resistance, connections from the voltage winding acrosssaid resistance, and connections having a serial relation to saidresistance for energizing the current winding in proportion to thecurrent traversing said resistance, said instrument connections eachincluding a rectifier and electrical isolating units for substantiallyeliminating direct load current from said windmgs.

7. In an electrical system including an alter nating-current circuittraversed by a load current and including an impedance of magnitudewhich is subject to variation, apparatus responsive to the magnitude ofsaid impedance, said apparatus comprising in combination with suchimpedance a ratio responsive instrument including a voltage winding anda current winding, connections from the voltage Winding across saidimpedance, and connections having a serial relation to said impedancefor energizing the current winding in proportion to the currenttraversing said impedance, a local source of direct measuring currentand connections for causing said direct measuring current to traversesaid impedance, each of said instrument connections including anisolating unit for minimizing the efiect of alternating load current insaid instrument.

8, An electric arc furnace system comprising in combination a supplycircuit, an arc furnace including an electrode, and an electrode holderconnected in the supply circuit, and a protective arrangement responsiveto predetermined changes in contact conditions between said electrodeand said electrode holder, said protective arrangement comprising incombination with such an electrode and its holder a voltage-currentratio responsive instrument including a voltage winding with a pair ofinput terminals and a current winding, connections from one of saidvoltage winding terminals to said electrode and from the other of saidterminals to said electrode holder, and connections having a serialrelation to said electrode holder supply circuit for energizing thecurrent winding in proportion to the current traversing said circuit.

9. In an electrical system including conductors in contact withconnections for causing passage of alternating current, a protectivearrangement responsive to predetermined changes in contact conditions,said arrangement comprising in combination with such contact aninstrument responsive to ratio of voltage and current including avoltage winding and a current winding, connections including atransformer operatively connecting the voltage Winding across the saidcontact, connections including.a rectifier having a serial relation tosaid contact and said altemating-current connections for energizing thecurrent winding in proportion to the alternating current traversing saidcontact, and a second rectifier interposed between said transformer andsaid voltage winding.

10. An electrical system comprising in combination a current-carryingcircuit and a protective arrangement therefor, said circuit including apair of conductors in contact and a useful load supplied by saidcircuit, said protective arrangement comprising in combination with suchcontact and connections for causing passage of load current through suchcontact, a device responsive to ratio of voltage to current having amovable element, said ratio responsive device including a voltagewinding and a current winding, connections bridging the voltage windingacross said 14 contact, and connections having a serial relation to saidcontact for energizing the current winding in proportion to the currenttraversing said circuit.

RAPHAEL SCHEIRMANN.

REFERENCES CITED 'Ihe following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 292,761 Olmsted Jan, 29, 1884528,268 Armen Oct, 30, 1894 1,089,814 Beach et a1 Mar. 10, 19141,214,763 Dixon Feb. 6, 1917 1,615,648 Pierce Jan. 25, 1927 1,697,188Keller et al. Jan. 1, 1929 1,779,347 Vawter Oct. 21, 1930 1,814,869Traver July 14, 1931 1,825,476 Gaarz et al. Sept. 22, 1931 1,839,148Green Dec. 29, 1931 1,919,079 St. Clair et al. July 18, 1933 1,923,565Austin Aug. 22, 1933 1,931,862 Felton Oct. 24, 1933 2,044,546 Ryan June16, 1936 2,057,845 Pattee Oct. 20, 1936 2,125,050 Josephs Jul 26, 19382,149,756 Aremberg et al. Mar. 7, 1939 2,218,629 Swart Oct. 22, 19402,221,556 Roemisch Nov. 12, 1940 2,232,715 Matthews Feb, 25, 19412,261,686 Kesselring Nov. 4, 1941 2,307,499 Frakes Jan. 5, 19432,316,170 Kesselring Apr. 13, 1943 2,324,458 Peters et a1 July 13, 1943FOREIGN PATENTS Number Country Date 426,238 Great Britain Mar. 29, 1935OTHER REFERENCES Laws, Electrical Measurements, first edition, 1917,pages 94-97 inclusive, Q 535 L3.

Radio World, Dec. 1936, pages 12 and 13.

